Rupture of atherosclerotic plaques in blood vessels disrupts endothelium and exposes collagen, a potent trigger for recruiting and aggregating platelets during thrombosis. A key signaling pathway in this process is the thrombin-mediated cleavage of protease-activated receptor 1 (PAR1) on platelets. Trivedi et al. found that matrix metalloprotease 1 (MMP-1) on platelets also cleaves PAR1, but at a site distinct from that cleaved by thrombin. Treatment of platelets with a peptide corresponding to the MMP-1 cleavage product of PAR1 activated the guanosine triphosphatase RhoA (which was blocked by the PAR1 antagonist RWJ-56110) and the p38 mitogen-activated protein kinase (MAPK). The inactive form of MMP-1 (pro-MMP-1) associated with the collagen receptor αIIbβ3 integrin. Like the MMP-1–generated PAR1 cleavage fragment, collagen treatment of platelets activated RhoA and p38 MAPK, which was blocked by PAR1 antagonists or the MMP-1 inhibitor FN-439. An in vitro assay in which anticoagulated blood was flowed over collagen-coated surfaces at rates that produced high shear stress conditions showed that pretreatment of platelets with either MMP-1 or PAR1 inhibitors did not affect their adhesion but decreased the initiation and propagation of thrombi, an effect not observed with thrombin inhibitors. Platelet activation in guinea pigs in response to direct injection of collagen was reduced by inhibition of MMP-1 or PAR1. FeCl3 injection causes injury in the carotid artery similar to that caused by rupture of atherosclerotic plaques, and eventually the artery becomes occluded because of thrombosis at the injury site. However, administration of either MMP-1 or PAR1 antagonists increased the time required for complete arterial occlusion. Furthermore, platelets in clots contained higher MMP-1 activity compared with platelets from uninjured animals or animals treated with FN-439 before FeCl3 injection. The authors propose that MMP-1 activation of PAR1 is required for the early stages of thrombosis and may be a potential therapeutic target for preventing arterial thrombosis.